Just some basic info on the hydroboost system from GM - might help someone down the line:
The Hydro-Boost system, see Figs. 1 and 2, provides an additional cylinder in the brake system. This cylinder contains no brake fluid. The Hydro-Boost cylinder is hydraulically operated, by pressurized fluid from the power steering pump, providing power assist to operate a dual master cylinder brake system.
The booster is composed of two sections; the linkage section and the power section.
The booster is designed so that if a total absence of power assist occurs, the brakes can be applied manually, but somewhat greater pressure on the brake pedal is required.
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Complaints about power brake operation should be handled as if two separate systems exist. Check for faults in the regular brake system first. If it is okay, start looking over the power brake circuit. For a quick check of proper power unit operation, press the brake pedal firmly and then start the engine. The pedal should fall away slightly and less pressure should be needed to maintain the pedal in any position. On vacuum suspended power units, air will rush into the air intake when the brakes are applied.
Another check begins with removal of the stop light switch and installation of a pressure gauge. Take a reading with the engine off and the power unit not operating. Maintaining the same pedal height, start the engine and take another reading. There should be a substantial pressure increase in the second reading.
Pedal-free travel and total travel are critical. As a general rule, brakes should be adjusted or relined if the pedal is closer than 1-1/2 in. from the floor with the brakes applied. Free travel should be kept strictly to specifications.
Take a manifold vacuum reading if the power unit isn't giving enough assistance. Remember, though, that some of the new V-8s have less than 15 in. of vacuum at idle. If manifold vacuum is abnormally low, tune the engine and then try the power brakes again. Naturally, loose vacuum lines and clogged air-intake filters will cut down brake efficiency. Most units have a check valve that retains some vacuum in the system when the engine is off. A vacuum gauge check of this valve will tell you when it is restricted or stuck open or closed.
A simple check of the hydraulic system should be made before proceeding. Loosen the connection between the power unit and the wheel cylinder lines. If the brakes release, the trouble is in the power unit hydraulic circuit. If the brakes still will not release, look for a restricted brake line or similar difficulties in the regular hydraulic circuit.
A residual pressure check valve is usually included immediately under the brake line connection on hydraulic-assist power brakes. This valve maintains a slight hydraulic pressure on the brake lines and wheel cylinders to give better pedal response. If it is sticking, the brakes may not release.
Power brakes that have a hard pedal are usually suffering from a milder form of the same ills that cause complete power unit failure. Collapsed or leaking vacuum lines or insufficient manifold vacuum, as well as punctured diaphragms or bellows and leaky piston seals, all lead to weak power unit operation. A steady hiss when the brake is held down means a vacuum leak that will cause poor power unit operation.
Do not immediately condemn the power unit if the brakes grab. First look for all the usual causes, such as greasy linings or scored drums. Then investigate the power unit.
Regardless of whether the brakes are vacuum or hydraulically assisted, certain general service procedures apply. Only top quality, clean brake fluid should be used in power brakes. More seals and valves are used with power brake systems than with ordinary brakes, so an inferior brake fluid will do much more damage. For the same reason, be sure all dirt is kept out of the system. Additionally, on models with hydraulic assist brakes, care must be taken not to mix the fluids of the booster hydraulic system and brake hydraulic system as the fluids are not compatible.
The fact that brakes will operate even if the power unit fails gives us a clue to successful power brake service. This means the conventional brake system is left intact and a power unit is simply added to the existing system. Troubleshooting is then exactly the same until we get to the power unit. As with conventional hydraulic brakes, a spongy pedal with power brakes still means air in the system and grease on the linings will still make the brakes grab. Keep in mind, however, that power brakes give a higher line pressure, thus making leaks more critical.
Power units do not require adjustment. Either they work or they don't. If they don't, the various valves and connections are simply replaced. The only exception is that the power units themselves sometimes have an adjustable connection to the brake pedal or linkage.
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